This invention relates to a method of seam tracking in electric arc welding processes. More particularly, the present invention relates to a method employing infrared sensors capable of directly viewing the seam without interference from the arc radiation itself.
In arc welding processes, it is desirable to be able to move the electric arc welding torch along the seam to be welded. Furthermore, it is desirable that this be done automatically by the arc welding system. At present, most automated welding processes are limited to spot welding operations. However, continued development of automated welding processes, particularly those processes carried out by general purpose manufacturing robots, require certain feedback control structures and, in particular, structures which not only indicate the quality of the weld that is being made, but which also indicate the lateral position of the torch arm relative to the seam to be welded. In certain situations, seam tracking movements could be programmed into the welding system for predetermined seam position, curvature and dimension. However, it is much more desirable to be able to provide an electric arc welding system with means for automatically tracking the torch arm along the seam to be welded. It is further desired that this process occur continuously and automatically and that it be essentially undisturbed by the specific shape of the seam being welded.
More particularly, in automated welding, it is necessary to control the path of the welding arm so that the weld puddle remains centered on the weld seam. Moreover, although the general configuration of the seam in repetitive jobs can be programmed, fit variations from part to part are often sufficient to cause substantial deviations of the weld path from individual seams, resulting in poor welds. Accordingly, sensors are highly desirable which can determine the relative position of the weld puddle and the seam directly ahead of it, thereby providing a control signal which can be used to center the puddle. It is further desirable that the sensor be accurate to several mils, have a time response on the order of seconds or faster, sense the relative seam puddle position as close to the puddle as possible, operate from a working distance of several inches from the weld, all under the constraint of having only limited access to the workpiece. Furthermore, the sensor should work reliably in many different weld geometries and positions and be small, light and economical.
Several different forms of seam tracking apparatus have already been applied to guide automated and robotic welders. Among these devices are the mechanical contact type, such as "Cecil gauges", electromagnetic and electrostatic sensors, television viewers with signal processing electronics, and optical sensors using laser sources. Furthermore, sensors based upon the variation of arc parameters as the arc is zig-zagged slightly across the seam have also been employed. However, each of these previously described sensors fails to satisfy one or more of the above-mentioned desirable criteria.
Other work in this field has been reported in a progress report titled "Improvement of Reliability of Welding by In-process Sensing and Control (Development of Smart Welding Machines for Girth Welding of Pipes)" submitted to the Department of Energy in June, 1981 by Jose Converti et al. This report describes initial experiments conducted using contact sensors (thermocouples) to probe the temperature distribution near the weld puddle and seam. Attempts to use near infrared photodiodes, described therein, for remote temperature sensing were not successful due to significant optical interference from plasma radiation reflected from the metal surface. In particular, Converti et al. propose using a simple optical filter to reduce the radiation from the plasma arc through the use of materials similar to conventional welders' goggles.
However, one of the instant inventors has discovered that radiation from a tungsten inert gas (T.I.G.) welding arc is typically confined to a band having infrared radiation bandwidths below approximately 3 microns. This discovery has indicated that infrared sensors may be employable to probe the true surface temperature in the immediate vicinity of the weld puddle. Furthermore, tests conducted by the instant inventors indicate that such sensors provide the requisite temporal and resolution characteristics required in automated seam tracking welding processes.